Microwave attenuator



Nov. 22, 1960 M. s. TANENBAUM ETAL 2,961,621

MICROWAVE ATTENUATOR Filed Nov. 21, 1958 2 Sheets-Sheet 1 -WHIIII III 2 FlG.'l

ENTORS HARRY R. PERlNl KJQLMR RNEY lNV MORTON S. TANENBAUM Nov. 22, 1960- M. s. TANENBAUM EIAL MICROWAVE ATTENUATOR 2 sheat s-Sheet 2 Filed Nov. 21, 1958 INVENTORS I MORTON S. TANENBAUM HARRY R. PERINI p [146 ORY United States Patent MICROWAVE ATIENUATOR Morton S. Tanenbanm, Massapeqna, and Harry R. Perini, Astoria, N.Y., assignors to Sperry Rand Qorporation, Great Neck, N.Y., a corporation of Delaware Filed Nov. 21, 1958, Ser. No. 775,555

8 Claims. (Cl. 333-81) This invention relates to attenuating devices and more particularly to attenuators for use with microwave transmission lines.

A type of microwave transmission line which of late is being used extensively is a strip transmission line. One form of such a line is comprised of a pair of ground plane conductors which lie in spaced parallel planes. Disposed between and parallel to the ground plane conductors is a strip conductor which is quite narrow as compared with the widths of the ground plane conductors. The strip conductor is separated from the ground plane conductors by low loss dielectric material. Electro-magnetic waves propagate between the strip conductor and ground plane conductors in a transverse electromagnetic mode. This type of line shall be referred to hereinafter as a double ground plane strip transmission line.

Another form of the strip transmission line is the single ground plane line wherein a narrow strip conductor is separated from just one wide ground plane conductor by a low loss dielectric material.

Attenuators for use in strip transmission lines have been constructed in the past by inserting a lossy material between the ground plane conductors, for example, and adjacent the strip conductor in order that the lossy material will lie in the electromagnetic field of the waves propagating between the conductors and thereby attenuate said waves. These attenuators, however, are frequency sensitive and introduce more attenuation to waves at higher frequencies than to waves at lower frequencies. As is known, this results from the fact that as the wavelengths of the waves decrease the length of the attenuating material through which the waves pass appears electrically longer. This makes accurate calibration of a variable attenuator very difiicult because at each setting of the attenuator, waves of difierent frequencies will be attenuated by different amounts.

It is therefore an object of this invention to provide a substantially frequency insensitive microwave attenuator.

It is another object of this invention to provide an improved variable attenuator for strip transmission lines.

A further object of this invention is to provide a substantially frequency insensitive variable attenuator for strip transmission lines.

It is a further object of this invention to provide a microwave attenuator which has a desired attenuating characteristic over a range of frequencies.

These and other objects of the invention which will become more apparent as the description proceeds are achieved in a double ground plane strip transmission line by movably positioning a member which is dissipative to electromagnetic waves adjacent the strip conductor in the path of waves propagating on the line. Disposed on the dissipative member are frequency sensitive impedance means which introduce a minimum amount of perturbation to waves propagating on the line with a frequency at the high end of a range of frequencies, and a substantially greater amount of perturbation to waves with a frequency at the low end of the range of frequencies. The frequency sensitive impedance means are arranged so that as the dissipative means introduces more attenuae tion into the line the frequency sensitive impedance means introduces more perturbation to the waves at the lower frequencies.

The present invention will be discussed by referring to the accompanying drawings wherein:

Fig. l is a plan view, partially cut away, of one embodiment of the present invention;

Fig. 2 is a sectional view taken at section 22 of Fig. 1;

Fig. 3 is a plan view of the frequency insensitive dis sipative member employed in one embodiment of the present invention.

Fig. 4 is a graph which is used to help explain the operation of the device of this invention; and

Fig. 5 is an illustration of another embodiment of the present invention.

Referring to Figs. 1 and 2, the attenuator constructed in accordance with this invention comprises first and second spaced, parallel ground plane conductors 11 and 12 which are made of a material such as copper which has good electrical conductivity. Positioned above ground plane conductor 11 is a sheet of low loss dielectric material 13. Disposed above dielectric member 13 is a narrow strip conductor 14 which has an arcuate section 15 along a portion of its length. Connector 16 provides means for coupling one end of the strip transmission line to a coaxial transmission line. Another connector, not shown, will be connected in similar manner to the opposite end 17 of strip conductor 14.

As is known to those skilled in the art, members 11, 13 and 14 may be a single unit constructed from a copper clad dielectric member such as Teflon or polystyrene, wherein the shape of strip conductor 14 is formed by any of the well known printed circuit techniques. It is also known that the dielectric material may be air, in which case the strip conductor is supported and spaced from the ground plane conductors at intervals along its length.

In a device constructed according to this invention, ground plane conductors 11 and 12 were made from relatively thick metallic plates to provide a mechanically rigid device.

Positioned above strip conductor 14 is a dissipative member 18 which is comprised of a centrally apertured dielectric disc. A portion of one plane surface of mem ber 18 is coated with a film of lossy material 19, Fig. 3, which is dissipative to electromagnetic waves. Dissipative film 19 extends over substantially one-half the surface of the disc. Dissipative member 18 is rotatable about its central axis, and in the position illustrated in Fig. 1 the lossy material 19 lies completely in the electromagnetic field of the waves propagating between ground plane conductors 11, 12, and the arcuate section 15 of strip conductor 14. When rotated from the position illustrated in Fig. 1, the dissipative film 19 is completely out of the electromagnetic field of said waves.

Angularly disposed on the portion of member 18 which is occupied by lossy material 19 are a plurality of radially extending conductive strip members 20. Strip members 20 may be strips of copper foil which are cemented onto the dissipative film 19. Conductive strip members 20 function as frequency sensitive impedance means to provide a frequency insensitive attenuator, as will be explained below, and may have elfective lengths either onehalf or one-quarter wavelength long for waves having a frequency at the high end of a given range of frequencies. The strips will be either open-circuited or shortcircuited at their ends nearest the center axis of member 18, depending upon their lengths. Conductive strip members 20 extend radially substantially to the outer circumferential boundary of arcuate section 15, Fig. 1.

In the embodiment of the invention illustrated in Figs. 1 and 2, strip members 20 each have an effective electrical length substantially one-quarter wavelength long for waves at the high end of a given frequency range, and are short-circuited by annular conductive shorting disc 21 which is coaxially positioned on member 18 so as to be in electrical contact with strip members 20. Therefore, the effective electrical lengths of strip members 20 will extend from their outer edges to the outer edge of shorting disc 21.

Disposed next to upper ground plane conductor 12 is dielectric member 22 which is similar to dielectric member 13.

Disposed over strip conductor 14 and dielectric member 13 is a circular sheet 27 of dielectric material such as Teflon. A similar sheet of dielectric material 28 is positioned on dielectric member 22 on the side adjacent dissipative member 18. Sheets 27 and 28 provide low friction bearing surfaces on which dissipative member 18 rotates, thus preventing excessive wearing of the dissipative film 19, Strip members 20 and strip conductor 14.

A rotatable shaft 23 extends through ground plane conductor 12, dielectric member 22, and has a radially extending flange 30 which is secured to dissipative member 18 by means of screws 31 which pass through dissipative member 18 and engage annular member 32. Screws 33 positioned around the periphery of the attenuator engage threaded holes in ground plane 11 and maintain the component parts in assembled relationship.

Shaft 23, and thus dissipative member 18, may be rotated by turning knob 24. An annular dial 26 provides means for indicating the relative position of dissipative member 18. Dial 26 may be calibrated directly in decibels of attenuation.

In the operation of the device of Figs. 1 and 2, a source of electromagnetic waves and a utilization device, neither shown, are coupled respectively to coaxial connector 16 and end 17 of strip conductor 14 to propagate transverse electromagnetic waves in a given range of frequencies along the strip transmission line comprised of ground plane conductors 11, 12, and strip conductor 14. When dissipative member 18 is rotated 180 from the position illustrated in Fig. l, waves at any frequency within a given range of frequencies will propagate along the line substantially unattenuated since lossy film 19 and conductive members 20 are positioned out of the electromagnetic field of those waves. As the knob 24 is turned, the portion of dissipative member 18 having the lossy material 19 disposed thereon will be introduced into the region adjacent strip conductor .14 and into the electromagnetic field of the waves, thus attenuating the waves. Because the waves at the upper end of the given frequency range have a shorter wavelength than the waves at the lower end of the frequency range the portion of dissipative member 18 which is effective to attenuate the waves is electrically longer to the higher frequency waves and they will suffer more attenuation than will the lower frequency waves. This discrepancy in attenuation between the waves of different frequencies is compensated for by conductive strip members 20, which are short-circuited by annular shorting disc 21, and which are effectively one-quarter wavelength long to waves having a frequency at the upper end of the given range of frequencies. Thus the members 20 will present very high impedances to the higher frequency waves, and because members 20 are electrically parallel coupled to the strip transmission line, they will introduce a minimum of perturbation to these high frequency waves. To waves at the lower end of the given range of frequencies strip members 20 will be electrically shorter than one-quarter wavelength and the impedances presented to the strip transmission line by conductive members 20 will be lower than were the impedances to the higher frequency waves.

These parallel coupled low impedances will cause perturbations in the incident lower frequency waves and will cause a portion of them to be reflected, which so far as the load is concerned, is the equivalent of adding lossy attenuation into the line. Because shorted conductive members 20 are frequency sensitive, the perturbations of the waves propagating on the strip transmission line, and the amount of incident energy which is reflected, will increase as the frequency of the waves decrease. Thus, additional loss to the lower frequency waves is introduced by strip conductors 20 and by choosing the correct number, sizes, and positions for conductive members 20. The loss suffered by the lower frequency waves may be made substantially equal to the loss suffered by the higher frequency waves as a result of lossy film 19. The number, sizes, and positions of conductive members 20 are best determined experimentally.

A 0 to 11 db variable attenuator designed to operate over a frequency range of 9.25 to 1225K me. was constructed substantially as illustrated in Figs. l-3, and had the following approximate dimensions and characteristics:

Diameter of ground plane conductors 11,

12 inches- 6% Width of center conductor 14 (copper foil) do .165 Radius of curvature of arcuate section 15 do 2.5 Diameter of dissipative member 18 do 6 Thickness of dissipative member 18 do .024 Lossy material 19 on one surface only of dissipative member 18.

Angular extent of lossy material 19 degrees Resistance of lossy material 19 ohms/square Number of conductive strip members 20 (copper foil) 3 Radial extent of conductive strip members 20 inches 2.58 Width of conductive strip members 20 do .082 Angular separation between conductive strip members 20 degrees 43.7

Diameter of conductive shorting disc 21 inches 3.25

In the operation of the device just described, with the dissipative member 18 in position for maximum attenuation, the difference in attenuation between waves at 9.25K me. and waves at 1225K me. was approximately 1.0 db. Without the use of conductive strip members 20 the difference in attenuation was approximately 3.25 db.

By employing more than three conductive strip members 20, the attenuation curves for waves throughout the range of frequencies may be made substantially identical.

It is evident from the above discussion that conductive strip members 20 function as shorting stubs parallel coupled to the strip transmission line. In the operation of the frequency insensitive attenuator just described it is desirable that the impedances presented to the strip transmission line by stub members 20 will always decrease as the frequency of the waves decrease. To 215 sure this, shorted strip members 20 may be made onequarter wavelength long and not some other odd multiple of a quarter wavelength to waves at the high end of the given range.

A more rapid rate of change of impedance with change in frequency may be achieved by employing stub members 20 which are an odd multiple of a quarter wavelength long to waves at the high end of the given range of frequencies. In this instance, however, care must be taken to assure that the lengths of the stubs do not become some multiple of one-half wavelength at the frequency of the waves to be propagated, since those waves will be substantially completely reflected under such conditions.

.Annularshorting disc 21 may be electrically connected to. one of the ground plane conductors 11 or 12 although this is not necessary. Because the impedance of a strip transmission line is an inverse function of the width of the strip conductor, annular disc 21 will constitute a relatively wide strip conductor between ground planes l1 and 12 and will constitute a section of line having a very low impedance. For practical purposes this section of very low impedance line terminating the strip conductors 24) is substantially equivalent to a true short circuit termination for the strip conductors 20.

The cooperative relationship between strip members 20 and dissipative member 18 to produce a substantially uniform calibration curve of rotation of dissipative member 18 vs. attenuation, irrespective of the frequency of the waves, may be demonstrated with the aid of the curves of Fig. 4.

Fig. 4 shows two curves which illustrate the attenuation introduced only by dissipative member 18 to Waves having a frequency at the high end of the frequency range, F and to waves having a frequency at the low end of the frequency range, F It will be seen that the slope of the Fhigh curve is greater than the slope of the F curve. Therefore, to raise the F curve up to coincide with the F curve, it will be necessary to introduce additional loss to the low frequency waves as dissipative member 18 is rotated further into the field of the low frequency waves. This is accomplished in the embodiment of the invention illustrated in Figs. 1-3 by angularly disposing conductive members 20 about the portion of dissipative member 18 occupied by lossy material 19. In this manner, as dissipative material 1? is rotated into the field of the low frequency waves more of the conductive members 20 will progressively come into energy coupling relationship with the waves propagating on the line and will progressively reflect a greater portion of the low frequency waves. These reflected waves will tend to propagate back toward the source, but again will propagate through dissipative material 19 and will be further attenuated. This latter feature inherently produces an attenuator having a low standing wave ratio since the reflected waves are attenuated by lossy material 19 and prevent large standing waves from being set up on the strip transmission line.

The construction and mode of operation just described will produce a relatively frequency insensitive attenuator. In some microwave systems, however, there may be other components which are frequency sensitive and it may be desirable that the attenuator have a frequency sensitive response which will compensate for the response of the other component so as to produce a resultant flat response. In such a case the lengths of conductive strip members 20 may be made something other than onequarter or one-half wavelength so that the attenuator may have a non-linear frequency response over a given range of frequencies.

Another embodiment of the attenuator of the present invention is illustrated in the simplified drawing of Fig. 5. The transmission line is a single ground plane strip transmission line comprised of a wide ground plane conductor 51 and a narrow strip conductor 52 separated by dielectric member 53. Strip conductor 52 has a section 54 disposed transversely of the longitudinal axis of the transmission line. Members 51, 52 and 53 may be constructed from a sheet of copper clad dielectric material and strip conductor 52 may be formed by a printed circuit technique. of lossy material 56 disposed thereon is secured to rod 57. Rod 57 is slidable in groove 58 for positioning dissipative member 55 into or out of the region adjacent the transverse section 54 of strip conductor 52, and thus into or out of the electromagnetic field of waves propagating on the strip transmission line. Rod 57 has a scale 60 marked on its upper surface which may be calibrated directly in db of attenuation. A plurality of spaced conductive strip members 59 are positioned on dissipative member 55 and extend perpendicularly to transverse SEQ- A dissipative member 55, having a fi'm 6 tion 54 and perpendicularly to'the direction of movement of dissipative member 55. Conductive members 59 may in this instance have electrical lengths substantially equal to one-half a wavelength for waves at the high end of the frequency ranges and may have open circuit terminations.

The operation of the device illustrated in Fig. 5 is substantially the same as the operation of the device illustrated in Figs. 1-3, except that the movement of dissipative member 55 is linear instead of rotary.

It is to be understood, however, that conductive members 59 may have lengths and terminations similar to the conductive members 20 of Figs. 1-3. Alternatively, conductive members 20 of Figs. 1-3 may be open circuited and may have lengths substantially equal to one-half wavelength at the high end of the given range of frequencies.

Also, the single ground plane strip transmission line illustrated in Fig. 5 may be a double ground plane transmission line as illustrated in Figs. 1-3 if desired, and vice versa.

Although the attenuator of the present invention has been described for use in a strip transmission line having a wide ground plane conductor, or conductors, and a narrow strip conductor, it may also be employed with other types of transmission lines such as coaxial lines,

hollow pipe waveguides, and parallel plate transmission 'lines.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

g 1. A microwave attenuator comprising a section of strip transmission line having at least a first ground plane conductor lying in a plane extending parallel to an axis, a strip conductor spaced from and extending parallel to said ground plane conductor for propagating electromagnetic waves within a given range of frequencies between said conductors, means dissipative to electromagnetic waves disposed adjacent said strip conductor in the path of said waves for attenuating waves propagating along said transmission line, said dissipative means being "characterized by attenuating waves at the upper end of said frequency range more than the waves at the lower end of said frequency range, and means for presenting a substantially constant attenuation to electromagnetic waves over said given range of frequencies comprising spaced frequency sensitive conductive strips disposed on said dissipative means in energy coupling relationship with waves propagating between said conductors, said frequency sensitive strips having such terminations and such electrical lengths as to present substantially no perturbation to waves at the high end of said frequency range propagating on said line, whereby the attenuation of said high frequency waves in passing through said attenuator is substantially only the attenuation produced by said dissipative means, said frequency sensitive conductive strips having such electrical lengths for waves at the lower end of said frequency range as to introduce perturbation to waves at the low end of said frequency range propagating on said line to reflect a portion of said lower frequency waves to further attenuate waves at the low end of said frequency range, the reflective attenuation produced by said frequency sensitive conductive strips being approximately equal to the difference in attenuation produced by said dissipative means between the Waves at the high and low ends of the given frequency range. v

2. The combination as claimed in claim 1 wherein said frequency sensitive strips are each substantially onequarter wavelength long for waves having a frequency at the upper end of said frequency range, each of said frequency sensitive strips being terminated substantially in a short circuit.

' 3. The combination as claimed in claim 1 wherein said frequency sensitive strips are each substantially one-half wavelength long for waves having a frequency at the upper end of said frequency range, each of said frequency sensitive strips being terminated substantially in an open circuit.

4. A frequency insensitive microwave attenuator comprising a section of strip transmission line adapted to propagate transverse electromagnetic waves in a given frequency range, said transmission line comprising at least a first wide ground plane conductor and a narrow strip conductor spaced from and extending parallel to said ground plane conductor, a member dissipative to clectromagnetic waves movably disposed adjacent said strip conductor in the path of waves propagating on said transmission line, said dissipative member being characterized by attenuating waves at the upper end of said frequency range more than the waves at the lower end of' said frequency range, said dissipative member being rotatable about an axis transversely displaced from said strip conductor, and means for presenting at a fixed position of said dissipative member a substantially constant attenuation to electromagnetic waves over said given range of frequencies comprising a plurality of radially extending conductive strip members disposed on said dissipative member in energy coupling relationship with waves propagating on said transmission line, each of said plurality of conductive strip members having such a termination and such an electrical length as to present a high impedance parallel coupled to waves propagating on said transmission line at a frequency at the high end of said frequency range so that said conductive strip members have substantially no effect on waves at the high end of said frequency range, said conductive strip members present ing a low impedance parallel coupled to waves at the low end of said frequency range propagating on said transmission line to thereby introduce perturbation to said low frequency waves to reflect a portion of said low frequency waves, the attenuation resulting from the reflection of a portion of said lower frequency waves being substantially equal to the difference in attenuation between waves at the high and low ends of the frequency range produced by said dissipative means, whereby said microwave attenuator produces a relatively constant attenuation over said given frequency range at a fixed position of said dissipative member.

5. A frequency insensitive microwave attenuator comprising a section of strip transmission line adapted to propagate transverse electromagnetic waves in a given frequency range, said transmission line comprising at least a first wide ground plane conductor and a narrow strip conductor spaced from and extending parallel to said ground plane conductor, said strip conductor having an arcuate section along a portion of its length, a disc shaped member disposed parallel to said ground plane conductor and adjacent said arcuate section in the path of waves propagating along said transmission line, said disc shaped member being rotatable about an axis transversely displaced from said arcuate section and having a portion thereof dissipative to electromagnetic energy, said dissipative portion being characterized by attenuating waves at the higher end of said frequency range more than the waves at the lower end of said frequency range, and means for presenting at a fixed position of said disc shaped member a substantially constant attenuation to electromagnetic waves over said given frequency range comprising a plurality of radially extending conductive strips angularly disposed on the dissipative portion of said disc thereby to act as stub transmission lines coupled to said strip transmission line when said dissipative member is in the path of said waves, each of said radially extending strips being terminated in substantially a short circuit at their re p c ve s s s a stsaid a d e ch h in an setrical length substantially equal to one-quarter wave length for waves having a frequency at the high end of said frequency range to present parallel coupled high impedance to waves at the high end of said frequency range propagating on said line, whereby waves at the high end of said frequency range propagate on said transmission line substantially unaffected by said conductive strips, said radially extending conductive strips having an electrical length substantially less than one-quarter Wave length for Waves at the lower end of said frequency range to present parallel coupled low impedance to said lower frequency waves propagating on said transmission line, whereby said conductive strips introduce a perturbation to said lower frequency waves and reflect a portion there of to provide additional attenuation of said lower frequency waves.

6. A frequency insensitive microwave attenuator comprising a section of strip transmission line adapted to propagate electromagnetic waves in a given frequency range along an axis, said transmission line comprising at least a first wide ground plane conductor and a narrow strip conductor spaced from and lying parallel to said ground plane conductor, said strip conductor having a section thereof disposed transversely of said axis, a member dissipative to electromagnetic waves movably disposed adjacent said transverse section of the strip conductor in the path of waves propagating along said transverse section, said dissipative member being movable along a path parallel to said transverse section, said dissipative member being characterized by attenuating waves at the high end of said frequency range more than the waves at the low end of said frequency range, means for presenting at a fixed position of said dissipative member a substantially constant attenuation to electromagnetic Waves over said given range of frequencies comprising a plurality of frequency sensitive conductive strip members disposed in spaced relationship on said dissipative member and extending perpendicularly to said transverse section of the strip conductor, said frequency sensitive strip members having such terminations and such electrical lengths as to present parallel coupled high impedances to waves at the high end of said frequency range propagating on said strip transmission line so that said conductive strip members have substantially no effect on Waves at the high end of said frequency range, said frequency sensitive conductive strip members presenting parallel coupled low impedances to waves at the low end of said frequency range propagating on said strip transmission line to thereby introduce perturbation to said low frequency waves to reflect a portion of said low frequency waves, at any fixed position of said dissipative member the attenuation resulting from the reflection of a portion of said lower frequency waves being substantially equal to the difference in attenuation produced by said dissipative means between waves at the high and low ends of the frequency range, whereby at a fixed position of said dissipative member said microwave attenuator produces a relatively constant attenuation over said frequency range.

7. A frequency insensitive microwave attenuator comprising a section of strip transmission line having at least a first ground plane conductor lying in a plane extending parallel to an axis, a strip conductor spaced from and extending parallel to said ground plane conductor for propagating electromagnetic waves within a given range of frequencies along said conductors, means dissipative to electromagnetic waves disposed parallel to said ground plane conductor and adjacent said strip conductor in the path of said waves for attenuating waves propagating along said transmission line, said dissipative means being characterized by attenuating waves at the high end of said frequency range more than the waves at the low end of said frequency range, and means for presenting parallel coupled high impedance to waves at the high and 9? s id f e uen r n e n p ral el o le low impedance to waves at the low end of said frequency range propagating on said line, said last-named means comprising a plurality of frequency sensitive conductive strips disposed on said dissipative means in the path of said waves so that said frequency sensitive conductive strips have substantially no effect on waves at the high end of the given frequency range, whereby the attenuation of said high frequency Waves is substantially only the attenuation produced by said dissipative means, said frequency sensitive conductive strips introducing a perturbation to waves at the low end of said frequency range and reflecting a portion of said low frequency waves thereby to introduce additional attenuation to the lower frequency waves propagating through said attenuator, the attenuation resulting from the reflection of a portion of said waves at the low end of said frequency range being approximately equal to the difference in attenuation produced by said dissipative means between waves at the high end and the low end of said frequency range, said dissipative means being movable with respect to said strip conductor to introduce more or less dissipative material into the path of said waves as said dissfpative means is moved to thereby vary the attenuation of Waves propagating on said line, said frequency sensitive conductive strips being disposed in spaced relationship along the direction of movement of said dissipative means to introduce an increasing number of frequency sensitive conductive strips into energy coupling relationship with waves propagating along said line as more dissipative material is introduced into the path of said waves.

8. A frequency insensitive microwave attenuator comprising a section of strip transmission line having at least a first ground plane conductor lying in a plane extending parallel to an axis, a strip conductor spaced from and extending parallel to said ground plane conductor for propagating electromagnetic waves within a given range of frequencies along said conductors, means dissipative to electromagnetic waves disposed parallel to said ground plane conductor and adjacent said strip conductor in the path of said waves for attenuating waves propagating along said transmission line, said dissipative means being characterized by attenuating waves at the high end of said frequency range more than the waves at the low end of said frequency range, and means for presenting parallel coupled high impedance to Waves at the high end of said frequency range and parallel coupled low impedance to waves at the low end of said frequency range propagating on said line, said last-named means comprising a plurality of frequency sensitive conductive strips disposed on said dissipative means in the path of said waves so that said conductive strips have substantially no effect on waves at the high end of the given frequency range, whereby the attenuation of said high frequency waves is substantially only the attenuation produced by said dissipative means, said frequency sensitive conductive strips introducing a perturbation to waves at the low end of said frequency range and reflecting a portion of said low frequency waves thereby to introduce additional attenuation to the lower frequency waves propagating through said attenuator, the attenuation resulting from the reflection of a portion of said waves at the low end of said frequency range being approximately equal to the difference in attenuation produced by said dissipative means be tween waves at the high end and the low end of said frequency range, whereby said attenuator produces a substantially constant attenuation over said given frequency range.

References Cited in the file of this patent UNITED STATES PATENTS 2,603,710 Bowen July 15, 1952 2,725,535 Crieg Nov. 29, 1955 OTHER REFERENCES Sanders Associates Handbook Of Tri-Plate Microwave Components, copyright Dec. 31, 1956, Sanders Associates, Nashua, New Hampshire. Pages 56 and 57 relied upon. 

